Identify a pane by its window and its pane index

By identifying a pane by both its window and pane index, `VimuxCloseRunner()` will always close the correct pane, even in case `VimuxCloseRunner()` is called from a Vim instance in a different window than the current window.

This fixes closing a pane in the current window from a `VimuxCloseRunner()` call executed in a Vim instance in a different window. I'm sending Vim commands to a Vim server from a Vimux runner, that is the reason why I encounter this issue.